Coal mills in power plants with direct firing systems for handling lignite, brown coal, hard coal and anthracite were designed for a defined coal flow range. This defined coal flow range into the power plant includes a minimum coal flow rate, below which the normal operation of the power plant would be hampered.
The FIG. 1 is a depiction of a common coal mill in a power plant 100 that uses a direct firing system for all forms of coal. The coal can be lignite, a brown coal, a hard coal or anthracite (hereinafter generically referred to as “coal”). In the FIG. 1, the coal mill comprises a beater wheel mill 102 in fluid communication with a furnace 106. Coal is charged to the mill 102 where it is dried and pulverized and then discharged to burners 110 and 112 where it is combusted in a furnace 106. Coal along with “drying and transport” gas (gas that is used to dry and transport the coal into the furnace) and gas that is used to control the temperature of the coal (“temperature control gas”) as it exits the mill 102, prior to being charged into the furnace 106 via the burners 110 and 112.
The drying and transport gas along with the temperature control gas are mixed in a mixing chamber 128 prior to entering the mill 102, where they are mixed with the pulverized coal. The coal is then combusted in the furnace 106, to generate heat and flue gases. The flue gases are discharged to the outside.
There are many different types of coals and each of these types of coals are generally fed to a different type of mill in order to be comminuted and combusted in the furnace. Table 1 documents the different types of coals and mills that these coals are used in. It also details the conditions in the mills.
TABLE 1Mill TypeBeaterCommonBallWheelBeaterImpactBowlTubeMillsMillsMillsMillsMillsMills are part ofdirect firing systemMill feedmore than one burner or one burner with more than one burner nozzleCoal/fuel typesAnthracite, hard coal, brown coal lignite and pulverized fuelsCoal/fuel moisture content0%-80%Mill Inlet Temperaturefrom ambient temperature upto 900° C.Mill Outlet Temperaturefrom ambient temperature upto 250° C.Drying and transport Media (gas)Flue gas, hot air, cold air, cold gasGas temperature control media (gas)hot air, cold air, cold gas, injection of water or steamVentilation of gas byitselfitself, oneitself, oneadditionaladditionaladditionaladditionalfanfanfan or afan or acombinationcombinationof bothof both
The FIG. 2 is another depiction of a coal mill in a power plant 100 that uses a direct firing system for coal. As with the mill of the FIG. 1, the coal mill comprises a beater wheel mill 102 in fluid communication with a furnace 106. Coal is charged to the beater wheel mill 102 where it is dried as detailed below and then discharged to burners 110, 112 and 114 where it is combusted in a furnace 106.
The coal along with flue gas, primary air, and optionally water and/or cold gas are charged to a beater wheel mill 102 to pulverize the coal. The flue gas, the primary air, the water and the cold gas are first mixed in a mixing chamber 128 and then discharged to the beater wheel mill 102.
The beater wheel mill 102 is in fluid communication with a classifier 104, which functions to separate coal particles above a desired size from other coal particles that are transferred to the furnace 106. The coal particles above the desired size are recycled to the mill to undergo further pulverization.
In the beater wheel mill 102, the incoming coal is caught by the rapidly circulating beater plates 103 which are fixed at the perimeter of the beater wheel and comminuted by the impact of the beater plates and after that against the armored mill housing. Beater wheel mills have a ventilating effect—they transport the pulverized coal and carrier gas to the main burners 110 and 112 and the vapor burners 114 (e.g., a lignite firing system with vapor separation). During the normal operation of a coal fed power plant with a beater wheel mill, about 40% of the total gas flow (along with about 20% of the coal from the beater wheel mill) takes place through the vapor burner 114, while about 60% of the total gas flow (along with about 80% of the coal from the beater wheel mill) takes place through the main burners 110 and 112.
The coal (which has a natural moisture content of 30 wt % to 75 wt %, based on the total weight of the coal) is charged into the beater wheel mill 102 along with recycled flue gas and/or water, cold gas and primary air. The recycled flue gas is at a temperature of about 1000° C. and is used to dry the coal. The temperature of the flue gas is reduced from about 1000° C. to about 400° C. before contacting the coal in the mill by blending the flue gases by the addition of the primary air (at a temperature of about 300° C.), cold gas (at a temperature of 170° C.) and water injection to the recycled flue gases prior to contacting the coal.
The heating of the coal (by the flue gases) with the resulting evaporation of moisture from the coal results in the reduction of the gas temperature to about 120 to about 250° C. as it is discharged from the mill to the classifier 104. Maintaining the temperature of the gas between about 120 to about 250° C. is useful because it reduces the possibility of damage to the mill from fire and/or explosions that occur at elevated temperatures greater than 250° C.
When the flow rate of coal into the mill is reduced in response to a lower demand for power it increases the possibility of explosion in the mill because reducing the amount of coal in the mill facilitates a reduction in the moisture content present in the mill, which prevents the proper reduction in gas temperature and coal temperature to about 120 to about 250° C.
In order to operate under lower demand for power (i.e., a reduced load demand) several different parameters can be varied. One possibility is to increase the amount of hot air, cold gas and water to the mill to compensate for the lower flow rate of the coal. Increasing the amount of hot air, cold gas and water controls the flue gas at the time it contacts the coal, which in turn facilitates controlling the temperature of the coal and gases being discharged from the mill 102 to the classifier 104 to be below 250° C.
In order to effect the changes listed above, several variables have to be accounted for. These are as follows. It is desirable for the oxygen concentration in the gas (after being discharged from the mill) to be 12 volume percent (e.g., in a wet condition) or less to prevent an explosion. The drying performance of the mill and the crushing performance are also to be taken into consideration to ensure that the appropriate amount of coal is discharged into the classifier and the furnace at the temperature of about 120 to about 250° C. The transport performance is also to be taken into consideration and this factor includes transportation without pulsation at the appropriate flow rate to the burner nozzles. The transportation rate includes a deposit free flow in the mill spiral and ducts. It is also desirable for the concentration ratio of pulverized coal to gas flow for safe ignition and combustion to lie within safe limits. Taking all of these factors into consideration, the average controlled load operation range for a beater wheel mill is between 50 to 100% of the full load operation.
With the increasing use of wind power and solar power for energy generation, there is a desire for reducing the coal flow below the prescribed minimum coal flow rate (i.e., below 50%). Wind power plants and solar power plants operate sporadically. For example, wind plants generate a large amount of power when there is a large amount of wind and solar plants generate a large amount of power when there is bright sunlight. However, this power is often generated when there is a low load on the power plant (i.e., there is no need for so much power). In order to compensate for the excess power generated by a wind power plant (or a solar power plant), it is desirable to reduce the power generated by a coal fed power plant that works in conjunction with the wind power plant and/or the solar power plant. When the power generated by the coal fed power plant is to be reduced to accommodate power generation by a wind or solar power plant, the aforementioned safe average controlled load operation range (of between 50% and 100%) is no longer sufficient.
It is therefore desirable to find new methods and devices for permitting a coal fed power plant to operate under low load conditions so that it can accommodate high power generation in cogenerating wind and/or solar power plants.